1. Field of the Invention
This invention relates to a process for placing concrete through a high head, which process is particularly suitable for efficiently and safely placing concrete onto the surface of a substantially vertical structure in the ground at a substantial depth below the ground surface, such as in the gap between a pressure pipe and surrounding earth for an underground hydraulic power plant, vertical shafts for gates, vertical shafts for elevators and the like, without causing any separation of cement and aggregates.
2. Description of the Prior Art
Heretofore, in order to place concrete on the surface of a vertical shaft, steel pipes or extensible chutes have been used for feeding concrete as long as the depth of the vertical shaft is 10 to 20 meters. If the vertical shaft is deeper than 10 to 20 meters, materials of the concrete tend to be separated while being fed through such a high head and the impact of the dropping of the concrete tends to cause various hazards, so that buckets with a capacity of 1 to 3 cubic meters are vertically moved by a crane installed at the top of the vertical shaft for conveying the concrete down to a hopper located at suitable portion of the shaft, and the concrete is then placed to desired portions through a chute connected to the lower end of the hopper.
When the span of the vertical travel of the bucket becomes 100 to 200 meters, the vertical travelling speed of the bucket is restricted for ensuring the safety of the operation. As a result, the speed of placing the concrete is reduced to 5 to 10 m.sup.3 /hour or less, and the efficiency of construction is considerably lowered. Besides, when a worker stays at the place of depositing the concrete for compacting, he is exposed to the risk of being hit by concrete particles or blocks which are separated from the bucket moving above him and dropping downwards. The process of concrete placing with vertically moving buckets has a further difficulty in that, due to the use of a crane installed at the top of the vertical shaft, it is very difficult to work in the space above the crane and to install any temporary structures thereabove during the concrete placing.